1. Field of the Invention
The invention relates generally to methods and apparatus for fabricating electronic devices, such as integrated circuits, on substrates. More particularly, the invention relates to methods and apparatus for processing substrates using an inductively coupled plasma system.
2. Background of the Related Art
A variety of processes are currently used in the fabrication of integrated circuits and other electronic devices. For example, processes such as chemical vapor deposition and physical vapor deposition are used to deposit various conductive, semiconductive and dielectric materials onto substrates. In addition, etching processes are used to remove various conductive, semiconductive and dielectric materials from substrates. Both deposition and etch processes can include the use of a plasma generated in a processing chamber. One etch process used to advantage is an inductively coupled plasma etch process which uses an inductive coil to deliver RF energy into the chamber to excite gases introduced into the chamber into a plasma state. A conventional inductively coupled plasma (ICP) etch chamber is typically operated at a pressure of about 2-40 millitorr. A substrate being processed is mounted on a support member connected to a source of RF bias voltage and spaced from and generally below the RF inductive coil. Plasma is struck in the processing gas by the application of RF power to the RF coil, and the positive gas ions created in the chamber are attracted to the negatively biased substrate being processed. Depending on the gases used, a physical etching, a reactive etching or a combination physical and reactive etching occurs to remove material from the surface of the substrate being processed.
However, inductively coupled plasma processes are not typically truly inductive processes even though an inductive coil arrangement is used. As with most inductively coupled plasma hardware designs, ostensibly a significant amount of RF power is capacitively coupled through the dielectric window to the plasma via the high RF voltages occurring in the inductive coil. Indeed, it has been some capacitive coupling Measurements have shown that the typical working values of this RF voltage are between about 1200 and 3400 V peak. The coupling of plasma to these high voltages can result in erosion of the dielectric window of the chamber through ion bombardment. In applications where the dielectric window is a ceramic, such as aluminum oxide, the erosion of this material creates aluminum contamination within the chamber.
Typically, upon inspection of the internal surfaces of a dome after processing, heavy film deposits are present in the center and edge regions of the dome, but are clean from deposition in the region directly beneath the inductive source coil. The deposition build-up occurs because as material is removed in an etch process, the material typically deposits on other surfaces within the chamber. The area directly beneath the coil is kept clean by bombardment of the surfaces by particles generated in the plasma and attracted to the surface by the capacitive field generated by the inductive coil. The edges of the regions where heavy deposition occurs can peel away from the dome, resulting in high levels of particle contamination both on substrates processed in the chamber as well as in the chamber itself.
It is believed that capacitive coupling of the RF power into the plasma has this undesired effect on the erosion of the chamber dome and on the measured aluminum contamination levels in the chamber. For fluorine-based plasmas, the reduction of the capacitive coupling between the coil and the plasma is found to reduce the level of aluminum contamination. One attempt to decrease this coupling to essentially zero by use of a grounded electrostatic or Faraday shield is described in U.S. Pat. No. 5,811,022, entitled xe2x80x9cInductive Plasma Reactorxe2x80x9d which issued on Sep. 22, 1998 and which is incorporated herein by reference.
Several problems can arise if a Faraday Shield, i.e., a grounded shield, is used.
If the shield is properly designed, the shield will effectively eliminate any capacitive coupling between the inductive coil and the gases in the chamber, thereby minimizing the ability to ignite the plasma using only the inductive source because a higher voltage breakdown is needed to initiate the plasma discharge using a purely inductive source. Additionally, complete elimination of all capacitive coupling between the plasma and source coil has been found to have a detrimental effect on plasma stability for certain electronegative processing gasses. This effect has been seen to increase the RF source power required to sustain plasma for some processing gases.
Therefore, there is a need for an ICP system which can ignite plasma using capacitive coupling of power into a chamber, can maintain plasma using inductive coupling and can minimize the generation of particles and other contamination within the chamber.
The invention generally provides an ICP etch system having a voltage distribution electrode (VDE) disposed between an inductive coil and a dielectric window (i.e., a dome or a lid) of a chamber. The VDE generally forms a radially slotted shield which conforms to the geometry of the dielectric window of the chamber to provide substantially full window coverage. The VDE is preferably electrically floating, i.e., electrically isolated from both the inductive coil and ground. When the VDE is electrically floating, the field due to capacitive coupling from the inductive coil is coupled to the shield and is distributed across the area of the VDE. However, the VDE can also be connected to ground via a switch or relay to provide a grounded shield which could be utilized to advantage as a Faraday shield. Additionally, the VDE could be connected to a power source to power the VDE in desired applications such as etching or chamber cleaning. Still further, the shield could be connected to ground via a circuit element of non-zero impedance to moderate the potential on the shield between ground potential and the potential present when the shield is floating.
In another aspect, the invention provides a voltage distribution electrode for use in an inductively coupled plasma chamber. The VDE is preferably adapted and configured to minimize eddy current losses and may include multiple fingers or conductors defining generally parallel gaps therebetween. The VDE can be switched between an electrically floating configuration, a grounded configuration or a powered configuration.
In another aspect, the invention provides a method of distributing (thereby reducing the current density) the capacitive coupling of a RF voltage delivered to an inductive coil into a chamber. The method generally includes providing an electrically floating VDE between an inductive coil and a dielectric window of a chamber to distribute the voltage coupled to the shield over a large area.